Allow implicit conversions in return statements.

ICSharpCode.Decompiler.Tests/CustomAttributes/S_CustomAttributeSamples.cs

@@ -264,7 +264,7 @@ namespace NamedInitializerPropertyType
 		{
 			get
 			{
-				return (Type)null;
+				return null;
 			}
 			set
 			{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/CompoundAssignmentTest.cs

@@ -73,13 +73,12 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		
 		private int[,] Array()
 		{
-			return (int[,])null;
+			return null;
 		}
 		
 		private unsafe int* GetPointer()
 		{
-//			return null;
+			return null;
-			return (int*)0u;
 		}
 		
 		public int GetIndex()

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -183,7 +183,7 @@ namespace ICSharpCode.Decompiler.CSharp
 		protected internal override Statement VisitReturn(Return inst)
 		{
 			IType targetType = currentFunction.IsAsync ? currentFunction.AsyncReturnType : currentMethod.ReturnType;
-			return new ReturnStatement(exprBuilder.Translate(inst.Value).ConvertTo(targetType, exprBuilder));
+			return new ReturnStatement(exprBuilder.Translate(inst.Value).ConvertTo(targetType, exprBuilder, allowImplicitConversion: true));
 		}
 
 		protected internal override Statement VisitYieldReturn(YieldReturn inst)